MEMS scanning mirror with trenched surface and I-beam like cross-section for reducing inertia and deformation

ABSTRACT

A micro-electro-mechanical system (MEMS) device includes a mirror having a top surface with trenches, a beam connected to the mirror, rotational comb teeth connected to the beam, and one or more springs connecting the beam to a bonding pad. The mirror can have a bottom surface for reflecting light. The mirror can include a top flange and a bottom flange joined by a web, wherein the top and the bottom flanges form the top and the bottom surfaces, respectively. The rotational comb teeth can have a tapered shape. Stationary comb teeth can be interdigitated with the rotational comb teeth either in-plane or out-of-plane. Steady or oscillating voltage difference between the rotational and the stationary comb teeth can be used to oscillate or tune the mirror.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/778,742,filed Feb. 13, 2004 now U.S. Pat. No. 7,046,421, which is incorporatedherein by reference.

FIELD OF INVENTION

This invention relates to micro-electro-mechanical system (MEMS)devices, and more particularly to MEMS scanning mirrors.

DESCRIPTION OF RELATED ART

Various electrostatic comb actuator designs for MEMS scanning mirrorshave been proposed. The extensive applications of these devices includebarcode readers, laser printers, confocal microscopes, projectiondisplays, rear projection TVs, and wearable displays. Typically a MEMSscanning mirror is driven at its main resonance to achieve a large scanangle. To ensure a stable operation, it is crucial to ensure the mirrorand its associated movable structure will vibrate in the desired modeshape at the lowest and main resonant frequency. In many applications,the mirror size has to be large and the mirror surface has to be flat toensure high optical resolution. The mirror vibration/scanning speed alsohas to be fast for many applications. It is known that when the mirrorsize and scanning speed increase, the mirror dynamic flatnessdeteriorates. Without a flat mirror surface, the scanning mirror is oflittle use to many applications. In addition, this main frequency has tobe separated far from other structural vibration frequencies to avoidpotential coupling between the desired and the undesired mode shapes.

The undesired structural vibrations will increase the mirror dynamicdeformation and result in degraded optical resolution. Furthermore, someof the structural vibration modes may cause the rotationally movable andstationary comb teeth to come into contact and break the actuator alltogether. Two or more structural vibration modes with close resonantfrequencies may be coupled to produce high vibration amplitude thatleads to hinge failure. Thus, an apparatus and a method are needed inthe design of MEMS scanning mirrors to effectively improve the vibrationstability at resonance, and to ensure optical resolution of thesedevices.

SUMMARY

In one embodiment of the invention, a MEMS device includes a mirrorhaving a top surface with trenches, a beam connected to the mirror,rotational comb teeth connected to the beam, and one or more springsconnecting the beam to a bonding pad. The mirror can have a bottomsurface for reflecting light. The mirror can include a top flange and abottom flange joined by a web, wherein the top and the bottom flangesform the top and the bottom surfaces, respectively. The rotational combteeth can have a tapered shape. The beam can be connected to the mirrorat multiple locations. Stationary comb teeth can be interdigitated withthe rotational comb teeth either in-plane or out-of-plane. A steady oroscillating voltage difference between the rotational and the stationarycomb teeth can be used to oscillate or tune the mirror.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C illustrate top views of the layers in a MEMS devicein one embodiment of the invention.

FIGS. 2A, 2B, and 2C illustrate top views of the layers in a MEMS devicein another embodiment of the invention.

FIG. 3 illustrates a deformation of a scanning mirror in one embodimentof the invention.

FIGS. 4A, 4B, 4C, 4D, 4E, 4F, and 4G illustrate a MEMS device in anotherembodiment of the invention.

FIGS. 4H, 4I, 4J, and 4K illustrate the MEMS device of FIG. 4A withdifferent power schemes in embodiments of the invention.

FIG. 5 illustrates process for manufacturing the device of FIG. 4A inone embodiment of the invention.

FIGS. 6A, 6B, 6C, 6D, 6E, and 6F illustrate a MEMS device in anotherembodiment of the invention.

FIGS. 6G, 6H, 6I, and 6J illustrate the MEMS device of FIG. 6A withdifferent power schemes in embodiments of the invention.

FIG. 7 illustrates process for manufacturing the device of FIG. 6A inone embodiment of the invention.

FIG. 8 illustrates comb teeth in one embodiment of the invention.

Use of the same reference numbers in different figures indicates similaror identical elements.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A illustrates a MEMS scanning mirror device 100 in one embodimentof the invention. Device 100 includes a top layer 100A (FIG. 1B) and abottom layer 100B (FIG. 1C).

Referring to FIG. 1B, top layer 100A includes rotational comb teeth 108that are connected on opposing sides of beam-like structures 103A and103B. Proximate ends of beams 103A and 103B are connected by multiplesupport attachments 102 to opposing sides of a scanning mirror 101. Inother words, each beam is connected at multiple locations to scanningmirror 101. The positions and the number of support attachments 102 canbe refined through finite element analysis to improve the vibrationstability and to minimize dynamic deformation of scanning mirror 101. Byimproving the vibration stability and reducing dynamic deformation ofscanning mirror 101 with support attachments 102, the optical resolutionof device 100 is improved.

Beams 103A and 103B are attached by eight serpentine springs/hinges 105Ato 105H to bottom layer 100B (FIG. 1C) in a distributed manner along therotational axis (e.g., the x-axis) of scanning mirror 101. Specifically,the distal end of beam 103A is connected by spring/hinge 105A to anchor104A, and the distal end of beam 103B is connected by spring/hinge 105Hto anchor 104H. Along their lengths, beam 103A is connected bysprings/hinges 105B to 105D to corresponding anchors 104B to 104D, andbeam 103B is connected by springs/hinges 105E to 105G to correspondinganchors 104E to 104G. In one embodiment, springs 105B to 105G arelocated within beams 103A and 103B. Anchors 104A to 104H are mounted tobottom layer 100B (FIG. 1C).

Top layer 100A may include stationary comb teeth 109. In one embodiment,stationary comb teeth 109 provide the electrostatic biasing force usedto increase the driving efficiency of the movable structure by tuningits modal frequency. In another embodiment, stationary comb teeth 109provide the electrostatic driving force to drive scanning mirror 101. Inyet another embodiment, stationary comb teeth 109 provide both theelectrostatic biasing force and the electrostatic driving force.

Referring to FIG. 1C, bottom layer 100B includes surfaces 106A to 106Hthat serve as anchoring surfaces for the movable structure in top layer100A (FIG. 1A). Specifically, anchors 104A to 104H are bonded tocorresponding surfaces 106A to 106H. Cavity 107 accommodates therotation of scanning mirror 101 without touching bottom layer 100B. Inone embodiment, stationary comb teeth 110 provide the electrostaticdriving force to drive scanning mirror 101. In another embodiment,stationary comb teeth 110 provide the electrostatic biasing force usedto increase the driving efficiency of the movable structure. In yetanother embodiment, stationary comb teeth 110 provide both theelectrostatic driving force and the electrostatic biasing force.Stationary comb teeth 108 and 110 are interdigitated with rotationalcomb teeth 108 when viewed from above.

As described above, springs 105A to 105H are distributed along beams103A and 103B. By carefully adjusting the distribution of the torsionaland translational stiffness of these springs, all modal frequencies ofthe movable structure can be effectively separated and the desiredrotational mode can be designed at the lowest resonance frequency. Sincethe main resonant frequency is the lowest and far apart from otherstructural modal frequencies, the mirror rotation driven by a sinusoidalAC voltage will not excite any other undesired vibration mode.

Using multiple springs, the maximum stress and strain on each individualspring are noticeably lower than conventional scanning mirror designssupported by only a pair of torsional beams. Therefore, the distributedspring design significantly improves the device reliability andincreases the rotational angle. In summary, the system reliability andthe servo and the optical performance are all improved with embodimentsof the invention.

FIG. 2A illustrates a MEMS scanning mirror device 200 in one embodimentof the invention. Device 200 includes a top layer 200A (FIG. 2B) and abottom layer 200B (FIG. 2C).

Referring to FIG. 2B, top layer 200A includes a mirror 201 connected bymultiple support attachments 202 to beams 203A and 203B. Mirror 201 andsupport attachments 202 are similar to those shown in FIG. 1B.Rotational comb teeth 208 are connected to one side of beams 203A and203B.

Beams 203A and 203B are connected by springs/hinges 205A to 205H tostationary surface 204 of top surface 200A in a distributed manner alongthe rotational axis of scanning mirror 201. Specifically, the distal endof beam 203A is connected by spring/hinge 205A to surface 204, and thedistal end of beam 203B is connected by spring/hinge 205H to surface204. Along their lengths, beam 203A is connected by springs/hinges 205Bto 205D to surface 204, and beam 203B is connected by springs/hinges205E to 205G to surface 204.

Referring to FIG. 2C, bottom layer 200B includes a cavity 207 thataccommodates the rotation of scanning mirror 201 without touching bottomlayer 200B. In one embodiment, stationary comb teeth 210 provide theelectrostatic driving force to drive scanning mirror 201. In anotherembodiment, stationary comb teeth 210 provide the electrostatic biasingforce used to increase the driving efficiency of the moving structure.In yet another embodiment, stationary comb teeth 210 provide both theelectrostatic driving force and the electrostatic biasing force.Stationary comb teeth 210 are interdigitated with rotational comb teeth208 when viewed from above.

FIG. 3 shows a typical mirror dynamic deformation of a mirror 301.Mirror 301 rotates along the x-axis, which points in or out of the page.The total mirror dynamic deformation 302 is shown. The x-axis and they-axis form a plane where the original mirror surface resides. Thez-axis is used to describe the mirror out-of-plane motion. The mirrordynamic deformation is a function of mirror thickness, scanningfrequency, mirror size, and rotation angle. The peak-to-peak dynamicdeformation has to be smaller than one fourth of the wavelength toprevent diffraction from limiting the optical performance of thescanning mirror. It is estimated that the proposed mirror attachmentstructures and methods shown in FIGS. 1A and 2A reduce the mirrordynamic deformation up to 50 percent.

FIG. 4A illustrates a MEMS scanning mirror device 400 in one embodimentof the invention. Device 400 includes a top layer 402 bonded atop butelectrically insulated from a bottom layer 404.

FIGS. 4B and 4C illustrate the details of top layer 402. Top layer 402includes a top mirror layer 406 having an oblong shape. Top mirror layer406 includes trenches/grooves 408 on its top surface. Trenches 408reduce the mass of top mirror layer 406, which in turn minimizes thetotal dynamic deformation. By minimizing the total dynamic deformation,the optical resolution of device 400 is improved. Although shown to runalong the entire top surface, trenches 408 may be most effective whenplaced along the outer perimeter of top mirror layer 406 away from arotational axis 414. As described later, trenches 408 can be etched atthe same time as other components by controlling their width so they arenot etched through top mirror layer 406. Alternatively, a shadow maskcan be used to protect top mirror layer 406 during etching to preventtrenches 408 from etching through. The positions and the number oftrenches 408 can be refined through finite element analysis. Gaps 409Aand 409B separate top mirror layer 406 from the surrounding componentsin top layer 402. As described later, the width of gaps 409A and 409B isdesigned to be greater than the widths of the gaps around more fragilecomponents so that any trapped gas can escape around top mirror layer406 instead of the fragile components during the etching process.

Opposing sides of top mirror layer 406 are connected by multiple supportattachments 410 to the proximate ends of beam-like structures 412A and412B. By connecting top mirror layer 406 at multiple locations to beams412A and 412B, the dynamic deformation of top mirror layer 406 isminimized. The positions and the number of support attachments 410 canbe refined through finite element analysis.

Opposing sides of beams 412A and 412B about a rotational axis 414 areconnected to rotational comb teeth 416. Rotational comb teeth 416 eachhas a tapered body that consists of an end rectangular section that hasa smaller cross-section than a base rectangular section. By reducing thesize and thus the weight of rotational comb teeth 416 at its ends, theinertia of the entire structure is reduced. By reducing the structuralinertia, the scanning speed can be increased or/and the driving voltagecan be reduced. In one embodiment, rotational comb teeth 416 provide theelectrostatic biasing force used to increase the driving efficiency ofthe movable structure by tuning its modal frequency. In anotherembodiment, rotational comb teeth 416 provide the electrostatic drivingforce to driver the mirror. In yet another embodiment, rotational combteeth 416 provide both the electrostatic biasing force and theelectrostatic driving force.

Beams 412A and 412B are connected by serpentine springs to bonding padsmounted atop bottom layer 404. Specifically, beam 412A has a distal endconnected by a serpentine spring 422-1 to a bonding pad 424, and amidsection connected by serpentine springs 422-2 and 422-3 to a bondingpad 426 formed within beam 412A. Similarly, beam 412B has a distal endconnected by a serpentine spring 428-1 to a bonding pad 430, and amidsection connected by serpentine springs 428-2 and 428-3 to a bondingpad 432 formed within beam 412B. Thus, beams 412A and 412B are connectedby springs in a distributed manner along rotational axis 414 of topmirror layer 406. Beams 412A and 412B may include holes 433 to reducetheir mass.

By carefully adjusting the distribution of the stiffness and location ofthe springs, all modal frequencies of the movable structure can beeffectively separated and the desired rotational mode can be designed atthe lowest resonance frequency. Since the main resonant frequency is thelowest and far apart from other structural modal frequencies, the mirrorrotation will not excite any other undesired vibration mode. By usingmultiple springs, the maximum stress and strain on each spring are lowerthan conventional scanning mirror designs supported by only a pair oftorsional beams. Since the stress and strain on each spring are reduced,the reliability of each spring is improved and the rotational angle isincreased.

Top layer 402 may include stationary comb teeth 434 that areinterdigitated in-plane with rotational comb teeth 416. Stationary combteeth 434 may have a tapered body like rotational comb teeth 416. In oneembodiment, stationary comb teeth 434 provide the electrostatic biasingforce used to increase the driving efficiency of the movable structureby tuning its modal frequency. In another embodiment, stationary combteeth 434 provide the electrostatic driving force to drive top mirrorlayer 406. In yet another embodiment, stationary comb teeth 434 provideboth the electrostatic biasing force and the electrostatic drivingforce. Stationary comb teeth 434 are connected to bond pad 436 mountedatop bottom layer 404.

FIGS. 4D, 4E, 4F, and 4G illustrate the details of bottom layer 404.Bottom layer 404 includes a bottom mirror layer 460 having a protrusion462 from an oblong plate 464. A gap 465 separates bottom mirror layer460 from the surrounding components in bottom layer 404. As shown inFIG. 4F, the bottom surface of plate 464 serves as the reflectingsurface and other structures can be aligned with the mirror usingassembly alignment marks 466 on the bottom surface of bottom layer 404.The top surface 467 of bottom mirror layer 460 is bonded with the bottomsurface of top mirror layer 406 to form the final mirror. As shown inFIG. 4G, the final mirror has an I-beam like structure where top mirrorlayer 406 forms the top flange, protrusion 462 forms the web, and plate464 forms the bottom flange. The I-beam like structure removes most ofthe mirror mass and stiffens the mirror structure. Therefore, itminimizes the dynamic deformation of the bottom mirror surface. Byminimizing the total dynamic deformation of bottom mirror surface, theoptical resolution of device 400 is improved. The shape of the I-beamlike structure can be refined through finite element analysis.

Bottom layer 404 includes surfaces for anchoring the bonding pads of themovable structure in top layer 402. Specifically, anchoring pads 468 and470 provide surfaces for mounting corresponding bonding pads 426 and432, and anchoring pad 472 provides a surface for mounting bonding pads424, 430, and 436.

Bottom layer 404 includes stationary comb teeth 474 that areout-of-plane interdigitated with rotational comb teeth 416. In otherwords, they are interdigitated when viewed from above or when the finalmirror is rotated. Stationary comb teeth 474 may have a tapered bodylike comb teeth 416 and 434. Referring to FIG. 4E, a gap 482 is providedbetween stationary comb teeth 474 and anchoring pad 472. Gap 482 has awidth greater than gaps 484 between adjacent stationary comb teeth 474so that gap 482 is etched deeper into bottom layer 404 than gaps 484. Adeeper gap 482 allows rotational comb teeth 416 to rotate at a greaterangle without contacting bottom layer 404. In one embodiment, stationarycomb teeth 474 provide the electrostatic driving force to drive thefinal mirror. In another embodiment, stationary comb teeth 474 providethe electrostatic biasing force used to increase the driving efficiencyof the movable structure. In another embodiment, stationary comb teeth474 provide both the electrostatic driving force and the electrostaticbiasing force. In yet another embodiment, the capacitance betweenrotational comb teeth 416 and stationary comb teeth 474 is sensed todetermine the rotational position of the mirror.

FIG. 5 illustrates a method 500 for making device 400 in one embodimentof the invention. The process starts at a step 0 with a silicon wafer502 having a silicon dioxide layer 504 formed on the top surface and asilicon dioxide layer 506 formed on the bottom surface. Wafer 502 isused to form bottom layer 404 (FIG. 4E) of device 400.

In step 1, a photoresist 508 is deposited on oxide layer 506, exposed,and developed in a lithographic process to define one or morelithographic alignment marks 511 (shown in step 3).

In step 2, the bottom surface of wafer 502 is etched to remove portionsof oxide layer 506 left unprotected by photoresist 508. In oneembodiment, oxide layer 506 is dry etched. The top surface of wafer 502is deposited with a photoresist 510 to protect it from the etching ofthe bottom surface.

In step 3, the bottom wafer surface of wafer 502 is etched to removeportions of wafer 502 left unprotected by oxide layer 506 to formlithography alignment marks 511. After the silicon dry etch, theremaining photoresists 508 and 510 are stripped.

In step 4, photoresist 510 is reapplied and is exposed and developed ina lithographic process to define bottom mirror layer 460 (FIG. 4E),surfaces 468, 470, and 472 (FIG. 4E), and stationary comb teeth 474(FIG. 4E) on the top surface of wafer 502. The mask used is aligned withthe lithographic alignment marks 511 on the bottom wafer surface.

In step 5, the top surface of wafer 502 is etched to remove portions ofoxide layer 504 left unprotected by photoresist 510. In one embodiment,oxide layer 504 is dry etched.

In step 6, the top surface of wafer 502 is etched to remove portions ofwafer 502 left unprotected by oxide layer 504 to form bottom mirrorlayer 460 (FIG. 4E), surfaces 468, 470, and 472, and stationary combteeth 474 (FIG. 4E). Afterwards, the remaining photoresist 510 isstripped and oxide layers 504 and 506 are removed by a wet or dry etch.

In step 7, a silicon wafer 512 is bonded to the top surface of wafer502. Wafer 512 has a silicon dioxide layer 514 formed on the top wafersurface and a silicon dioxide layer 516 formed on the bottom wafersurface. Wafer 512 is used to form top layer 402 (FIG. 4C) of device400. In one embodiment, wafers 512 and 502 are bonded by silicon fusion.

In step 8, a photoresist 518 is deposited on oxide layer 514, exposed,and developed in a lithographic process to define the components of toplayer 402 (FIG. 4C). The mask used is aligned with lithographicalignment marks 511 on the bottom wafer surface. Also defined in step 8are one or more lithographic alignment marks 521 (shown in step 10) anda separation trench 519 (shown in step 10). In order to etch trenches408 (FIG. 4C), which are etched into wafer 512 at a particular depth,along with the gaps that surround the other components, which are etchedthrough wafer 512, the dimensions of trenches 408 and the gaps of theother components are differentiated.

In step 9, the top surface of wafer 512 is etched to remove portions ofoxide layer 514 left unprotected by photoresist 518. In one embodiment,oxide layer 514 is dry etched. Afterwards, the remaining photoresist 518is stripped.

In step 10, the top surface of wafer 512 is etched to remove portions ofwafer 512 left unprotected by oxide layer 514 to form the components oftop layer 402 (FIG. 4C). In one embodiment, wafer 512 is etched using aDRIE process down to the etch stop formed by oxide layer 516. When thetop of device 400 is etched through, gas trapped between the bondedwafers 502 and 512 may escape and damage fragile components such as thecomb teeth. To prevent such damage, gaps 409A and 409B (FIG. 4C) aroundtop mirror layer 406 (FIG. 4C) are designed to be larger than the gapsaround the other components so that oxide layer 516 beneath gaps 409Aand 409B is etched through before the other gaps. This allows the air toescape around top mirror layer 406, which is a structurally strongcomponent.

In step 11, the top surface of the mirror is protected by a shadow masksurface 522 to prevent the top mirror layer 406 from being etchedthrough. This step is optional if inertia-reducing trenches 408 have awidth that is smaller than other gaps so they are not etched through.However, the shadow mask may be preferred to create inertia-reducingtrenches 408 having greater width, thereby removing more mass andfurther reducing the inertia of the rotating structure.

In step 12, a photoresist 520 is deposited on the bottom surface ofwafer 502, exposed, and developed on the bottom surface of wafer 502 todefine assembly alignment marks 466 (FIG. 4F), separation trench 509(shown in step 13), and gap 465 (FIG. 4E) for separating bottom mirrorlayer 460 (FIG. 4E) from bottom layer 404 (FIG. 4E). The mask used isaligned with lithographic alignment marks 521 on the top wafer surface.

In step 13, the bottom surface of wafer 502 is etched to remove portionsof wafer 502 left unprotected by photoresist 520 to form assemblyalignment marks 466 (FIG. 4F) and separation trench 509, and to separatebottom mirror layer 460 (FIG. 4E) from layer 404 (FIG. 4E). In oneembodiment, wafer 502 is etched using a DRIE process.

In step 14, portions of oxide layer 516 are removed from the structureto release the various components of device 400 while maintaining thebonds between the corresponding bonding and anchoring pads. In oneembodiment, portions of oxide layer 516 are removed using a hydrofluoricacid wet etch.

In step 15, the bottom surface of bottom mirror layer 460 (FIG. 4F) isdeposited with a reflective material (e.g., aluminum) to create a mirrorsurface. In one embodiment, a shadow mask is used to define areas to bedeposited with the reflective material.

In step 16, devices 400 made from wafers 502 and 512 are singulated. Inone embodiment, wafers 502 and 512 are singulated by dicing throughseparation trenches 509 and 519 (shown in step 15).

Referring to FIG. 4A, the operation of device 400 in one embodiment isexplained hereafter. Rotational comb teeth 416 are coupled via bondingpad 424 to receive a reference voltage from a voltage source 476 (e.g.,ground). Stationary comb teeth 434 are coupled via bonding pad 436 toreceive a steady voltage from a voltage source 478 (e.g., a DC voltagesource). Stationary comb teeth 474 (FIGS. 4D and 4E) are coupled viabonding pad 472 to receive an oscillating voltage from a voltage source480 (e.g., an AC voltage source). Thus, a steady voltage differencebetween rotational comb teeth 416 and stationary comb teeth 434 changesthe natural frequency of device 400, whereas an AC voltage differencebetween rotational comb teeth 416 and stationary comb teeth 474 (FIGS.4D and 4E) oscillates the mirror at the desired scanning frequency andat the desired scanning angle.

Referring to FIG. 4H, the operation of device 400 in another embodimentis explained hereafter. Rotational comb teeth 416 are coupled viabonding pad 424 to receive a steady voltage from voltage source 476(e.g., a DC voltage source). Stationary comb teeth 434 are coupled viabonding pad 436 to receive an oscillating voltage from AC voltage source480. Stationary comb teeth 474 (FIGS. 4D and 4E) are coupled via bondingpad 472 to receive a steady voltage from DC voltage source 478. Betweenrotational comb teeth 416 and stationary comb teeth 434, a steadyvoltage difference changes the natural frequency and the rotationamplitude of device 400 while an AC voltage difference oscillates themirror at the desired scanning frequency and at the desired scanningangle. Furthermore, a steady voltage difference between rotational combteeth 416 and stationary comb teeth 474 (FIGS. 4D and 4E) can also beused to change the amplitude of the rotational angle of device 400. Thecapacitance between rotational comb teeth 416 and stationary comb teeth474 can also be sensed through respective bonding pads 436 and 472 todetermine the rotational angle of device 400.

Referring to FIG. 4I, the operation of device 400 in another embodimentis explained hereafter. Rotational comb teeth 416 are coupled viabonding pad 424 to receive an oscillating voltage from AC voltage source480. Stationary comb teeth 434 are coupled via bonding pad 436 toreceive a steady voltage from DC voltage source 476. Stationary combteeth 474 (FIGS. 4D and 4E) are coupled via bonding pad 472 to receive asteady voltage from DC voltage source 478. Between rotational comb teeth416 and stationary comb teeth 434, a steady voltage difference changesthe natural frequency and the rotation amplitude of device 400 while anAC voltage difference between rotational comb teeth 416 and stationarycomb teeth 434 oscillates the mirror at the desired scanning frequencyand at the desired scanning angle. A steady voltage difference betweenrotational comb teeth 416 and stationary comb teeth 474 (FIGS. 4D and4E) can also be used to change the amplitude of the rotational angle ofdevice 400. The capacitance between rotational comb teeth 416 andstationary comb teeth 474 can also be sensed through respective bondingpads 436 and 472 to determine the rotational angle of device 400.

Referring to FIG. 4K, the operation of device 400 in another embodimentis explained hereafter. Rotational comb teeth 416 are coupled viabonding pad 424 to receive an oscillating voltage from AC voltage source480A. Stationary comb teeth 434 are coupled via bonding pad 436 toreceive a steady voltage from DC voltage source 476. Stationary combteeth 474 (FIGS. 4D and 4E) are coupled via bonding pad 472 to receivean oscillating voltage from AC voltage source 480B. Between rotationalcomb teeth 416 and stationary comb teeth 434, a DC voltage differencechanges the natural frequency and the rotation amplitude of device 400while an AC voltage difference oscillates the mirror at the desiredscanning frequency and at the desired scanning angle. Between rotationalcomb teeth 416 and stationary comb teeth 474 (FIGS. 4D and 4E), a DCvoltage difference can also be used to change the amplitude of therotational angle of device 400 while an oscillating voltage differencecan also be used to oscillate the mirror at the desired scanningfrequency and at the desired scanning angle. The capacitance betweenrotational comb teeth 416 and stationary comb teeth 474 can also besensed through respective bonding pads 436 and 472 to determine therotational angle of device 400.

Referring to FIG. 4I, the operation of device 400 in another embodimentis explained hereafter. Rotational comb teeth 416 are coupled viabonding pad 424 to receive a steady voltage from DC voltage source 476.Stationary comb teeth 434 are coupled via bonding pad 436 to receive anoscillating voltage from an AC voltage source 480A. Stationary combteeth 474 (FIGS. 4D and 4E) are coupled via bonding pad 472 to receivean oscillating voltage from an AC voltage source 480B. The oscillatingvoltage provided by AC voltage source 480B is out of phase (e.g., 180degrees out of phase) with the oscillating voltage provided by voltagesource 480A. Between rotational comb teeth 416 and stationary comb teeth434, a steady voltage difference changes the natural frequency and therotation amplitude of device 400 while an AC voltage differenceoscillates the mirror at the desired scanning frequency and at thedesired scanning angle. An AC voltage difference between rotational combteeth 416 and stationary comb teeth 474 (FIGS. 4D and 4E) can also beused to oscillate the mirror at the desired scanning frequency and atthe desired scanning angle. The capacitance between rotational combteeth 416 and stationary comb teeth 474 can also be sensed throughrespective bonding pads 436 and 472 to determine the rotational angle ofdevice 400.

FIG. 6A illustrates a MEMS scanning mirror device 600 in one embodimentof the invention. Device 600 includes a top layer 602 bonded atop butelectrically insulated from a bottom layer 604.

FIGS. 6B and 6C illustrate the details of top layer 602. Top layer 602includes a mirror 606 having an oblong shape. The bottom surface ofmirror 606 serves as the reflecting surface. The top surface of mirror606 includes trenches/grooves 608A, 608B, 608C, and 608D. Trenches 608Aare formed along the top outer perimeter of mirror 606 while trenches608B are formed along the bottom outer perimeter of mirror 606. Trenches608C and 608 are formed on the midsection of mirror 606. Trenches 608A,608B, 608C, and 608D reduce the mass of mirror 606, which in turnminimizes the dynamic deformation of mirror 606. By minimizing dynamicdeformation of mirror 606, the optical resolution of device 600 isimproved. As described later, trenches 608 can be etched at the sametime as other components by controlling their width so they are notetched through mirror 606. Alternatively, a shadow mask can be used toprotect mirror 606 during etching to prevent trenches 608 from etchingthrough. The mirror mass and inertia can be further reduced after thefabrication process by laser trimming. This method can adjust the mirrornatural frequency. The effective place to remove the mirror mass is thearea around the top and bottom outer perimeters of mirror 606.Therefore, areas on mirror 606 can be reserved for the laser trimmingprocess.

As described later, trenches 608 can be etched at the same times asother components by controlling their width so they are not etchedthrough mirror 606. The trenches were designed to remove the mirror massaround the mirror tips and outer diameter. This will effectively reducethe mirror inertia and reduce the mirror dynamic deformation. Thepositions and the number of trenches 608 can be refined through finiteelement analysis. Gaps 609A and 609B separate mirror 606 from thesurrounding components. As described later, the width of gaps 609A and609B is designed to be greater than the widths of gaps around morefragile components so that any trapped gas can escape around mirror 606instead of the fragile components during the etching process.

Opposing sides of mirror 606 are connected by multiple supportattachments 610 to the proximate ends of beam-like structures 612A and612B. By connecting mirror 606 at multiple locations to beams 612A and612B, the dynamic deformation of mirror 606 is minimized. The positionsand the number of support attachments 610 can be refined through finiteelement analysis.

Opposing sides of beams 612A and 612B about a rotational axis 614 areconnected to rotational comb teeth 616. Rotational comb teeth 616 eachhas a tapered body having an end rectangular section that has a smallercross-section than a base rectangular section. By reducing the size andthus the weight of rotational comb teeth 616 at its end, the inertia ofthe entire structure is reduced. By reducing the structural inertia, thescanning speed can be increased or/and the driving voltage can bereduced. In one embodiment, rotational comb teeth 616 provide theelectrostatic biasing force used to increase the driving efficiency ofthe movable structure by tuning its modal frequency. In anotherembodiment, rotational comb teeth 616 provide the electrostatic drivingforce to driver mirror 606. In yet another embodiment, rotational combteeth 616 provide both the electrostatic biasing force and theelectrostatic driving force.

Beams 612A and 612B are connected by serpentine springs to bonding padsmounted atop bottom layer 604. Specifically, beam 612A has a distal endconnected by a serpentine spring 622-1 to a bonding pad 624, and amidsection connected by serpentine springs 622-2 and 622-3 to a bondingpad 626 formed within beam 612A. Similarly, beam 612B has a distal endconnected by a serpentine spring 628-1 to a bonding pad 630, and amidsection connected by serpentine springs 628-2 and 628-3 to a bondingpad 632 formed within beam 612B. Thus, beams 612A and 612B are connectedby springs in a distributed manner along rotational axis 614 of topmirror 606. Beams 612A and 612B may include holes 633 to reduce theirmass.

By carefully adjusting the distribution of the location and stiffness ofthe springs, all modal frequencies of the movable structure can beeffectively separated and the desired rotational mode can be designed atthe lowest resonance frequency. Since the main resonant frequency is thelowest and far apart from other structural modal frequencies, the mirrorrotation will not excite any other undesired vibration mode. By usingmultiple springs, the maximum stress and strain on each spring are lowerthan conventional scanning mirror designs supported by only a pair oftorsional beams. Since the stress and strain on each spring are reduced,the reliability of each spring is improved and the rotational angle isincreased.

Top layer 602 may include stationary comb teeth 634 that areinterdigitated in-plane with rotational comb teeth 616. Stationary combteeth 634 may have a tapered body like rotational comb teeth 616. In oneembodiment, stationary comb teeth 634 provide the electrostatic biasingforce used to increase the driving efficiency of the movable structureby tuning its modal frequency. In another embodiment, stationary combteeth 634 provide the electrostatic driving force to drive mirror 606.In yet another embodiment, stationary comb teeth 634 provide both theelectrostatic biasing force and the electrostatic driving force.Stationary comb teeth 634 are connected to bonding pad 636 mounted atopbottom layer 404.

FIGS. 6D, 6E, and 6F illustrate the details of bottom layer 604. Bottomlayer 604 includes an opening 665 that accommodates the rotation ofmirror 606 without touching bottom layer 604. As shown in FIG. 6F, thebottom surface of bottom layer 604 includes assembly alignment marks 666for aligning other structures with mirror 606.

Bottom layer 604 includes surfaces for anchoring the bonding pads of themovable structure in top layer 602. Specifically, anchoring pads 668 and670 provide a surface for mounting bonding pads 626 and 632, andanchoring pad 672 provides a surface for mounting bonding pads 624, 630,636.

Bottom layer 604 includes stationary comb teeth 674 that areinterdigitated out-of-plane with rotational comb teeth 616. In otherwords, they are interdigitated when viewed from above. Stationary combteeth 674 may have a tapered body like comb teeth 616 and 634. Referringto FIG. 6E, a gap 682 is provided between stationary comb teeth 674 andanchoring pad 672. Gap 682 has a greater width than gaps 684 betweenadjacent stationary comb teeth 674 so that gap 682 is etched deeper intobottom layer 604 than gaps 684. A deeper gap 682 allows rotational combteeth 616 to rotate at a greater angle without contacting bottom layer604. In one embodiment, stationary comb teeth 674 provide theelectrostatic driving force to drive the final mirror. In anotherembodiment, stationary comb teeth 674 provide the electrostatic biasingforce used to increase the driving efficiency of the movable structure.In another embodiment, stationary comb teeth 674 provide both theelectrostatic driving force and the electrostatic biasing force. In yetanother embodiment, the capacitance between rotational comb teeth 616and stationary comb teeth 674 is sensed to determine the rotationalposition of the mirror.

FIG. 7 illustrates a method 700 for making device 600 in one embodimentof the invention. The process starts at a step A with a silicon wafer702 having a silicon dioxide layer 704 formed on the top wafer surfaceand a silicon dioxide layer 706 formed on the bottom wafer surface.Wafer 702 is used to form bottom layer 604 of device 600.

In step B, a photoresist 708 is deposited on oxide layer 706, exposed,and developed in a lithographic process to define one or morelithographic alignment marks 711 (shown in step D).

In step C, the bottom surface of wafer 702 is etched to remove portionsof oxide layer 706 left unprotected by photoresist 708. In oneembodiment, oxide layer 706 is dry etched. The top surface of wafer 702is deposited with a photoresist 710 to protect it from the etching ofthe bottom surface.

In step D, the bottom surface of wafer 702 is etched to remove portionsof wafer 702 left unprotected by oxide layer 706 to form lithographicalignment marks 711.

In step E, photoresist 710 is exposed and developed in a lithographicprocess to define surfaces 668, 670, and 672 (FIG. 6E), and stationarycombs 674 (FIG. 6E) on the top surface of wafer 702. The mask used isaligned and exposed with the lithographic alignment marks 711 on thebottom wafer surface.

In step F, the top surface of wafer 702 is etched to remove portions ofoxide layer 704 left unprotected by photoresist 710. In one embodiment,oxide layer 704 is dry etched.

In step G, the top surface of wafer 702 is etched to remove portions ofwafer 702 left unprotected by oxide layer 704 to form anchoring pads668, 670, and 672 (FIG. 6E), and stationary combs 674 (FIG. 6E). In oneembodiment, wafer 702 is etched using a deep reactive ion etching (DRIE)process. Afterwards, the remaining oxide layers 704 and 706 are removed.

In step H, a silicon wafer 712 is bonded to the top surface of wafer702. Wafer 712 has a silicon oxide layer 714 formed on the top wafersurface and a silicon dioxide layer 716 formed on the bottom wafersurface. Wafer 712 is used to form top layer 602 (FIG. 6C) of device600. In one embodiment, wafers 712 and 702 are bonded by silicon fusion.

In step I, a photoresist 718 is deposited on oxide layer 714, exposed,and developed in a lithographic process to define the components of toplayer 602 (FIG. 6C). Also defined in step I are one or more lithographicalignment marks 721 (shown in step K) and a separation trench 719 (shownin step K) In order to etch inertia-reducing trenches 608 (FIG. 6C),which are etched into wafer 712 at a particular depth, along with thegaps that surround the other components, which are etched through wafer712, the dimensions of trenches 608 and the gaps are differentiated. Themask used is aligned and exposed with lithographic alignment marks 711on the bottom wafer surface.

In step J, the top surface of wafer 712 is etched to remove portions ofoxide layer 714 left unprotected by photoresist 718. In one embodiment,thermal oxide layer 714 is dry etched.

In step K, the top surface of wafer 712 is etched to remove portions ofwafer 712 left unprotected by oxide layer 714 to form the components oftop layer 602 (FIG. 6C), alignment marks 721 and separation trench 719.In one embodiment, wafer 712 is etched using a DRIE process down to theetch stop formed by oxide layer 716. When the top of device 600 isetched through, gas trapped between the bonded wafers 702 and 712 mayescape and damage fragile components such as the comb teeth. To preventsuch damage, gaps 609A and 609B (FIG. 6C) around mirror 606 (FIG. 6C)are designed to be larger than the gaps around the other components sothat oxide layer 716 beneath gaps 609A and 609B is etched through beforethe other gaps. This allows the air to escape around mirror 606, whichis a structurally strong component.

In step L, the top surface of the mirror is protected by a shadow masksurface 722 to prevent the top mirror layer from being etched through.This step is optional if inertia-reducing trenches 608 have a width thatis smaller than other gaps so they are not etched through. However, theshadow mask may be preferred to create inertia-reducing trenches 608having greater width, thereby removing more mass and further reducingthe inertia of the rotating structure.

In step M, a photoresist 720 is deposited on the bottom surface of wafer702, exposed, and developed on the bottom wafer surface of wafer 702 todefine opening 665 (FIG. 6E) for mirror 606 (FIG. 6C). The mask used isaligned and exposed with lithographic alignment marks 721 on the topsurface.

In step N, the bottom surface of wafer 702 is etched to remove portionsof wafer 702 left unprotected by photoresist 720 to form opening 665(FIG. 6E). In one embodiment, silicon wafer 702 is etched using a DRIEprocess. Afterwards, the remaining photoresist 720 is stripped.

In step O, portions of oxide layer 716 are removed from the structure torelease the various components of device 600 while maintaining the bondsbetween the corresponding bonding and anchoring pads. In one embodiment,portions of oxide layer 716 are removed using a hydrofluoric acid wetetch.

In step P, the bottom surface of mirror 606 (FIG. 6C) is deposited witha reflective material (e.g., aluminum) to create a mirror surface. Inone embodiment, a shadow mask is used to define areas to be depositedwith the reflective material.

In step Q, devices 600 made from wafers 702 and 712 are singulated. Inone embodiment, wafers 702 and 712 are singulated by dicing throughseparation trenches 709 and 719.

Referring back to FIG. 6A, the operation of device 600 in one embodimentis explained hereafter. Rotational comb teeth 616 are coupled viabonding pad 624 to receive a reference voltage from a voltage source 676(e.g., ground). Stationary comb teeth 634 are coupled via bonding pad636 to receive a steady voltage from a voltage source 678 (e.g., a DCvoltage source). Stationary comb teeth 674 (FIGS. 6D and 6E) are coupledvia bonding pad 672 to receive an oscillating voltage from a voltagesource 680 (e.g., an AC voltage source). Thus, a steady voltagedifference between rotational comb teeth 616 and stationary comb teeth634 changes the natural frequency and the rotation amplitude of device600, whereas an AC voltage difference between rotational comb teeth 616and stationary comb teeth 674 (FIGS. 6D and 6E) oscillates the mirror atthe desired scanning frequency and at the desired scanning angle.

Referring to FIG. 6G, the operation of device 600 in another embodimentis explained hereafter. Rotational comb teeth 616 are coupled viabonding pad 624 to receive a steady voltage from a DC voltage sourceground 676. Stationary comb teeth 634 are coupled via bonding pad 636 toreceive an oscillating voltage from AC voltage source 680. Stationarycomb teeth 674 (FIGS. 6D and 6E) are coupled via bonding pad 672 toreceive a steady voltage from DC voltage source 678. Between rotationalcomb teeth 616 and stationary comb teeth 634, a steady voltagedifference changes the natural frequency and the rotation amplitude ofdevice 600 while an AC voltage difference oscillates the mirror at thedesired scanning frequency and at the desired scanning angle. A steadyvoltage difference between rotational comb teeth 616 and stationary combteeth 674 (FIGS. 6D and 6E) can be used to change the amplitude of therotational angle of device 600. The capacitance between rotational combteeth 616 and stationary comb teeth 674 can also be sensed throughrespective bonding pads 636 and 672 to determine the rotational angle ofdevice 600.

Referring to FIG. 6H, the operation of device 600 in another embodimentis explained hereafter. Rotational comb teeth 616 are coupled viabonding pad 624 to receive an oscillating voltage from AC voltage source680. Stationary comb teeth 634 are coupled via bonding pad 636 toreceive a steady voltage from DC voltage source 676. Stationary combteeth 674 (FIGS. 6D and 6E) are coupled via bonding pad 672 to receive asteady voltage from DC voltage source 678. Between rotational comb teeth616 and stationary comb teeth 634, a steady voltage difference changesthe natural frequency and the rotation amplitude of device 600 while anAC voltage difference oscillates the mirror at the desired scanningfrequency and at the desired scanning angle. A steady voltage differencebetween rotational comb teeth 616 and stationary comb teeth 674 (FIGS.6D and 6E) can be used to change the amplitude of the rotational angleof device 600. The capacitance between rotational comb teeth 616 andstationary comb teeth 674 can also be sensed through respective bondingpads 636 and 672 to determine the rotational angle of device 600.

Referring to FIG. 6J, the operation of device 600 in another embodimentis explained hereafter. Rotational comb teeth 616 are coupled viabonding pad 624 to receive an oscillating voltage from AC voltage source680A. Stationary comb teeth 634 are coupled via bonding pad 636 toreceive a steady voltage from DC voltage source 676. Stationary combteeth 674 (FIGS. 6D and 6E) are coupled via bonding pad 672 to receivean oscillating voltage from AC voltage source 680B. Between rotationalcomb teeth 616 and stationary comb teeth 634, a DC voltage differencechanges the natural frequency and the rotation amplitude of device 600while an AC voltage difference oscillates the mirror at the desiredscanning frequency and at the desired scanning angle. Between rotationalcomb teeth 616 and stationary comb teeth 674 (FIGS. 6D and 6E), a DCvoltage difference can also be used to change the amplitude of therotational angle of device 600 while an oscillating voltage differencecan also be used to oscillate the mirror at the desired scanningfrequency and at the desired scanning angle. The capacitance betweenrotational comb teeth 616 and stationary comb teeth 674 can also besensed through respective bonding pads 636 and 672 to determine therotational angle of device 600.

Referring to FIG. 6I, the operation of device 600 in another embodimentis explained hereafter. Rotational comb teeth 616 are coupled viabonding pad 624 to receive a steady voltage from DC voltage source 676.Stationary comb teeth 634 are coupled via bonding pad 636 to receive anoscillating voltage from an AC voltage source 680A. Stationary combteeth 674 (FIGS. 6D and 6E) are coupled via bonding pad 672 to receivean oscillating voltage from an AC voltage source 680B. The oscillatingvoltage provided by AC voltage source 680B is out of phase (e.g., 180degrees out of phase) with the oscillating voltage provided by voltagesource 680A. Between rotational comb teeth 616 and stationary comb teeth634, a steady voltage difference changes the natural frequency and therotation amplitude of device 600 while an AC voltage differenceoscillates the mirror at the desired scanning frequency and at thedesired scanning angle. An AC voltage difference between rotational combteeth 616 and stationary comb teeth 674 (FIGS. 6D and 6E) can also beused to oscillate the mirror at the desired scanning frequency and atthe desired scanning angle. The capacitance between rotational combteeth 616 and stationary comb teeth 674 can also be sensed throughrespective bonding pads 636 and 672 to determine the rotational angle ofdevice 600.

FIG. 8 illustrates comb teeth having another shape in one embodiment ofthe invention. Rotational comb teeth 816 each has a triangular body thattapers from the base to the end. By reducing the size and thus theweight of rotational comb teeth 816 at its ends, the inertia of theentire structure is reduced. By reducing the structural inertia, thescanning speed can be increased or/and the driving voltage can bereduced. Stationary comb teeth 834 and the stationary comb teeth in thelower layer can have the same triangular shape.

Various other adaptations and combinations of features of theembodiments disclosed are within the scope of the invention. Numerousembodiments are encompassed by the following claims.

1. A method for making a MEMS mirror device, comprising: etching a firstwafer to form: a bottom mirror layer comprising a web protruding from abottom flange; an anchoring pad; bonding a second wafer having a bottomoxide layer atop the first wafer; etching the second wafer to form: atop mirror layer comprising a top flange and trenches on the top flange,the top flange being bonded atop the web of the bottom mirror layer toform a mirror; a beam connected to the top mirror layer; a first springconnecting the beam to a first bonding pad, the first bonding pad beingbonded atop the anchoring pad; a plurality of rotational comb teethconnected to the beam; a first plurality of stationary comb teethconnected to a second bonding pad, the second bonding pad being bondedatop the anchoring pad.
 2. The method of claim 1, wherein said etchingthe second wafer comprises concurrently etching the trenches and gapsaround other components in the second wafer, the trenches having asmaller width than the gaps around the other components in the secondwafer.
 3. The method of claim 1, wherein said etching the second wafercomprises: concurrently etching the trenches and gaps around othercomponents in the second wafer; placing a shadow mask over the topmirror layer; and etching through the gaps around the other componentsin the second wafer.
 4. The method of claim 1, wherein said etching thesecond wafer comprises etching through a gap around the top mirror layerbefore etching through gaps around other components in the second wafer.5. The method of claim 1, wherein said etching a first wafer furtherforms a first plurality of stationary comb teeth.
 6. The method of claim5, wherein said etching a first wafer comprises etching a first gaparound the first plurality of stationary comb teeth deeper than a secondgap between adjacent teeth in the first plurality of stationary combteeth.
 7. The method of claim 1, further comprising etching through agap around the bottom mirror layer to allow the mirror to rotate.
 8. Themethod of claim 7, further comprising wet etching the bottom oxide layerto release bonded components between the first and the second wafers. 9.The method of claim 1, further comprising laser trimming the top mirrorlayer.
 10. The method of claim 1, further comprising forming areflective layer on the bottom flange.